Although aluminum is ordinarily considered as having a bright appearance, it often presents a dull or matte-like surface finish resulting from the operations employed to shape it into useful articles. Such operations include rolling, casting, forging, extrusion and the like. For some uses, a substantially mirror-like surface finish is desirable. As used herein, the term "mirror-like" means that the surface has a high distinctness of image ("D/I" for brevity). D/I is the sharpness of a reflected image as measured by the ratio of reflectance at 0.3.degree. from specular to the reflectance at the specular angle, that is, EQU D/I=(R.sub.S -R.sub.0.3)/R.sub.s .times.100%
wherein R.sub.s is the specular reflectance and R.sub.0.3 is the reflectance at 0.3.degree. from the specular angle. D/I=0 for a perfect diffuser and D/I=100 for a perfect mirror. As used herein, the term "substantially mirror-like" refers to a sheet surface having a D/I of at least 75% and preferably at least 80%.
Numerous chemical and electrochemical solutions have been developed in the prior art for polishing the surfaces of aluminum articles. However, none of the prior art solutions is completely satisfactory for its intended purpose. For example, one problem with prior art electrobrightening solutions is that they often produce a surface that is highly directional. As used herein, the term "directional" means that surface appearance of an electropolished aluminum alloy article is adversely affected by fine streaks in the rolling direction of the metal.
Askin et al. U.S. Pat. No. 5,417,819 states that an AA 5657 alloy aluminum plate was electrobrightened using a solution called ELECTROPOL 100, commercially available from Albright & Wilson of Richmond, Va. The ELECTROPOL 100 solution contains phosphoric acid and sulfuric acid, but not salts of sodium or potassium.
Bartkowski et al. U.S. Pat. No. 5,102,508 discloses a method of producing colored surfaces on aluminum automobile parts. In one example, an extruded section of an Al--Mg or Al--Mg--Si alloy article was mechanically ground and polished, then degreased and cleaned. The section was then subjected to electrolytic alkaline brightening in order to obtain a bright, reflective surface. The electrolyte contained 120 g/L trisodium phosphate, 330 g/L sodium carbonate, 10 g/L aluminum phosphate and beechwood extracts. Electrolysis conditions were 3 amp/dm.sup.2 current density, 70.degree.-80.degree. C. operating temperature and 18 minutes exposure time. Additional processing steps resulted in a gold-bronze color or various other shades of bronze.
Wernick et al. described a Brytal process developed in 1936 in their book entitled The Surface Treatment and Finishing of Aluminum and its Alloys (1987). The Brytal electropolishing process employed an alkaline solution containing 12-20 wt. % anhydrous sodium carbonate and 2.5-7.5 wt. % trisodium phosphate. Bath temperature was 75.degree.-90.degree. C. and voltage was 7-16 volts.
A principal objective of the present invention is to provide an electrobrightening process for aluminum alloy articles that results in a less directional surface appearance than prior art electrobrightening processes.
A related objective of the invention is to provide an electrobrightening bath for the process of the invention.
Additional objectives and advantages of our invention will become apparent from the following detailed description.